The present invention relates to a new process for the production of molded articles having gross densities above 900 kg/m.sup.3. In this process, the urea groups required for obtaining the superior mechanical properties are produced mainly by the reaction of aromatic polyisocyanates with aromatic amines which are obtained in situ from aromatic polyisocyanates and water.
It is known to produce molded articles from polyurethanes containing urea groups by reacting a reaction mixture of aromatic polyisocyanates, relatively high molecular weight polyhydroxyl compounds and aromatic diamines containing primary or secondary amino groups in closed molds using the reaction injection molding technique (see U.S. Pat. No. 4,218,543 or German Offenlegungsschrift No. 3,147,736). The molded articles obtained by this method have excellent mechanical properties due to their high urea group content. In the known art processes, the urea groups (which are necessary for obtaining these mechanical properties), are incorporated in the polyurethane structure by using aromatic diamines. In the known art processes, however, the above-mentioned advantage of the excellent mechanical properties is obtained at the expense of having to use expensive, special aromatic diamines, such as 1-methyl-3,5-diethyl-2,4-diaminobenzene or mixtures thereof with 1-methyl-3,5-diethyl-2,6-diaminobenzene. These diamines react very vigorously with aromatically bound isocyanate groups, with the result that the highly reactive reaction mixtures of the known art must be introduced into the molds within a very short time. For filling large molds, this can only be achieved by using special, high power dosing apparatus.
It was therefore an object of the present invention to provide a new process for the production of molded articles based on polyisocyanates having a gross density above 900 kg/m.sup.3 (DIN 53 420), in which the use of such generally expensive aromatic diamines is either no longer necessary or in comparatively very minor quantities, so that the advantages of the known art processes can be obtained without the abovementioned disadvantages.